While this change is not 100% required now, it's good habit
and we are checking for it since Moodle 4.4.
All the changes in this commit have been applied automatically
using the moodle.PHPUnit.TestReturnType sniff and are, exclusively
adding the ": void" return types when missing.
As far as now all them have correct privacy level2 namespace:
- Move them to "privacy" subdir.
- Rename the files to "provider_test.php", this includes old
privacy_test.php and privacy_provider_test.php files
- Rename the testcase to provider_test too (to match file name)
Also, change some relative paths and comments to point to new
locations.
